1. Field of the Invention
The present invention relates to a stabilization method of nano-sized emulsion prepared by using lecithin as an emulsifier and to an external application for skin containing the stabilized nano-sized emulsion.
2. Description of the Prior Art
The skin, as the primary protect shield of the human body, shields the internal organs from the potentially damaging stimuli such as environmental changes, ultra violet rays, pollutants, etc. Recently, a lot of efforts have been undertaken to suppress aging of the skin and to maintain healthy and beautiful skin. For example, as an effort to maintain skin function and to suppress the aging and melanin accumulation of the skin, physiologically active materials obtained from animals, plants and microorganisms have been used as components of cosmetic compositions.
Especially, percutaneous methods for absorbing effective components directly through the skin have been much studied. Such percutaneous absorbing method is described below.
As a basic method, the physiologically active material is transferred into the skin by dissolving the active material in a suitable solvent and applying the solution to the skin. Therefore, the appropriate solvent for dissolving the active material should be selected. However, there are some problems that selecting the solvents complying with the active materials is difficult and that the selected solvents also cause irritation. Further, because it is difficult to control the usability, there are some difficulties in formulation of the cosmetic.
Following the above dissolving method, an emulsion-type percutaneous releasing agent to improve the usability and skin absorption has been developed. The technology has been developed from an early method of containing the active agent into micrometer-sized emulsion particles to a method of containing the active agent into the nanometer-sized emulsion particles. Specifically, a technology for preparing nanometer- or micrometer-sized emulsion particles using useful agents and lipids, glycerol, water, phospholipid or water-soluble non-ionic surfactants is disclosed in U.S. Pat. No. 5,338,761. Preparing nano-sized particles using a charged-lipid as an emulsifier is disclosed in U.S. Pat. No. 6,120,751. Further, a method for preparing nano-sized particles using micro-emulsions, that are obtained when three phases consisting of emulsifier, oil and water become balanced, is disclosed in U.S. Pat. No. 5,152,923, WO 91/06286 and WO 91/06287.
However, when an unstable active agent is contained in the emulsion particle, because the emulsion membrane kinetically equilibrates with the outer phase, the active agent continuously contacts the water, which causes oxidation and decomposition of the particles. Therefore, a lot of emulsifiers are needed to contain a sufficient amount of active agents, which may cause skin irritation
To overcome the above problems, lecithin which has an excellent biocompatibility is used as an emulsifier in the preparation of nano-sized emulsion particles (U.S. Pat. Nos. 5,152,953 and 5,658,988).
However, when lecithin is used, because the lecithin has low physicochemical stability, the stability of the nano-sized emulsion particles prepared by using it becomes lower. Further, the stability of the active agents contained in the nano-sized emulsion particles is also low due to the instability of nano-sized emulsion particles.
The present inventors have conducted extensive studies on the method for stabilizing nano-sized emulsion particles using lecithin as emulsifier. As a result, they found that the lecithin-based nano-sized emulsion particles prepared by using tocopheryl derivatives together can achieve the stability of the emulsion particle itself, and also improve the stability of active agents contained in nano-sized emulsion particles. Based on this finding, the present invention is accomplished.
Thus, the purpose of the present invention is to provide a stabilization method of nano-sized emulsion particles prepared by using lecithin as emulsifier.
Further, another object of the present invention is to provide a nano-sized emulsion paticles stabilized by the foregoing method.
Also, still other object of the present invention is to provide an external composition for applying to the skin that contains the nano-sized emulsion particles.
The above and other objects and features of the present invention will be apparent to the skilled in the art from the following detailed description.